1. Field of the Invention
This invention relates generally to pumps and more specifically to rod-drawn subterranean pumps for use in wells such as oil and water wells.
2. Description of the Prior Art
Subterranean pumping apparatus of the prior art have typically included a cylindrical housing suitable for operational disposition at the bottom of a well, such as an oil well or a water well. In a common type of pumping apparatus, a rod string extends into the pump housing to support a plunger within the housing. The rod string is reciprocated to move the plunger in alternating upward and downward strokes.
A stationary valve, which is typically located at the bottom of the pump, defines with the housing and the plunger, a pump cavity. The stationary valve permits the fluid in the well to enter the pump chamber on the upward stroke of the plunger and inhibits the flow of the fluid from the chamber during the downward stroke of the plunger.
During the upward stroke of the plunger, the standing valve opens to permit gas and liquid from the well to enter the pump chamber. On the downward stroke of the plunger, the gas and liquid in the pump chamber is trapped due to the closing of the standing valve. As the plunger moved downwardly within the housing, the column of fluid in the region above the plunger provides a significant back pressure which maintains the traveling valve within the plunger in a closed state. With both the traveling valve and the stationary valve in a closed state, the downward progression of the plunger compresses the gas within the pump chamber.
At this point, one of two problems has typically occurred in the pumps of the prior art. If the gas in the pocket is compressed to a pressure of sufficient magnitude to overcome the back pressure of the column of fluid on the check valve, the gas in the pocket will create a severe impack on the check valve forcing it to open. This severe impact causes an upward compressive force, commonly referred to as "gas pound, " on the rod string which is particularly damaging to the well equipment.
If the gas enters the pocket at a relatively low pressure, the plunger may effectually reach the surface of the fluid before the check valve in the plunger opens. The resulting impact between the plunger and the liquid in the pump chamber also produces an upward compressive force, commonly referred to as "liquid pound, " on the rod string. The liquid pound is even more damaging than the gas pound since the liquid in the pump chamber is relatively incompressible and hence the magnitude of impact between the plunger and the surface of the liquid is even greater.
In an attempt to overcome this problem, a third valve has been added to the pump. This third valve is typically a "ring" valve which includes a valve housing having a fixed relationship with the housing of the pump. The rod string extends through the valve housing and a ring valve element, which extends circumferentially of the rod string, seats on the valve housing to support the column of fluid above the plunger during the downstroke of the rod string. Since the traveling check valve no longer supports the falling column of the liquid, the back pressure on the traveling valve is significantly reduced. As a result, the traveling valve will typically open during the downward stroke prior to the occurrence of gas pound or liquid pound.
Unfortunately, this two-stage pump has merely transferred the shock load to the upward stroke. At the bottom of the downward stroke, after a portion of the fluid in the pump chamber has passed through the traveling valve in the plunger, said fluid is supported above the plunger and below the third valve. If the plunger and the fluid between the plunger and the third valve are moving upwardly at a relatively high velocity, a severe shock load results when they encounter the closed ring valve and the back pressure provided by the column of fluid above the ring valve. This impact creates a significant tensile strength on the rod string which can be as damaging as the compressive stress previously described.